High speed semiconductor applications require accurate control over output impedance of a circuit. Voltage outputs typically require an output impedance that is matched to an electrical transmission line and a load that the circuit is driving. Current outputs typically require a high output impedance to consistently operate with a variety of loads. In the latter case, advances in CMOS (complementary metal-oxide-semiconductor) technologies make achieving high output impedances more difficult because of the short-channel effects in transistors, such as channel length modulation. In CMOS technology, transistors are used to realize logic functions, and in high speed and current mode circuits, FETs (field effect transistors) are used as current sources. In addition, scaling of the power supply voltages sometimes precludes stacking of multiple transistors to achieve a higher output impedance such as in a cascode current source. Alternatively, active impedance regulation may be used, but such regulation has speed limitations due to the feedback control loop. Therefore, a designer using a conventional circuit topology has to reconcile conflicting considerations that include tradeoffs between speed and impedance.
Another performance consideration is the non-linear characteristics of a load, such as a laser, driven by a transconductance amplifier (TCA). In most cases, the bandwidth capability of a waveguide exceeds that of a laser. Thus, in order to minimize cost per unit of data, it is advantageous to modulate lasers at their maximum frequency. However, operating lasers near their maximum frequencies causes a variety of non-ideal transient behaviors that can cause inter-symbol interference (ISI), which unfortunately increases the error rate.